The passage of time can mean the accumulation of Macs and email accounts. Many of us have been online now for long enough that we have stacked up digital detritus around us that’s sometimes split across old and new computers and offline drives. It may be frustrating when you want to—for example—search across email messages from several years ago, or even decades ago, and they aren’t readily available on your newest machine.
You can consolidate messages from multiple computers or backups of old ones without much fuss, though depending on the quantity of email, it might take some time. You can also pull in email from IMAP servers to store it locally to make sure it’s backed up as you wish and more readily searchable.